In voltage and current source inverters, control of the DC-bus voltage or current is required to control the balance between AC and DC power. In single phase systems control options are limited by ripple in the DC-bus voltage or current that results from ripple or harmonics present on the AC side. For example, in utility grid-connected inverters, such as in distributed generation (DG) systems, the ripple in the DC-bus voltage or current is at twice the grid frequency. The ripple can interact with the DC-bus voltage or current controller, resulting in distortion of the current delivered to the utility grid.
Conventional DC-bus voltage and current control methods are limited in terms of speed of transient recovery. Furthermore, improvements in speed, achieved by increasing the bandwidth of the control method, are not practical due to the presence the ripple across the DC-bus voltage or current. The bandwidth provided by conventional DC-bus voltage control methods is usually limited in order to prevent the double frequency ripple from propagating to the current control loop, which can cause the current delivered to the utility grid to become distorted. Furthermore, many commonly used control methods are proportional-integral (P)-based, which introduces the additional difficulty of integration. Fast integration can lead to saturation and instability, because large DC values are being integrated. This necessitates that all PI-based controllers are designed to provide slow integration, which again limits the speed of the control method with respect to transient recovery. However, PI-based control methods are well-known, easy to design, and provide simplicity, and accordingly they are often used for regulating the DC-bus voltage.
The limitations of prior DC-bus voltage control methods are twofold: (1) speed is limited by the bandwidth, which in turn is limited by the double frequency ripple across the DC-bus voltage, and (2) the simplicity provided by using conventional PI-based controllers comes at the price of further limiting speed due to the slow integrative action of the controller. Fast integration by PI-based controllers causes saturation when dealing with large DC values, so a PI-based control method necessarily provides slow integration. Both limitations affect the speed of the control method. Nonlinear controllers can provide better bandwidth than linear controllers, however they are generally not preferred because they tend to be more complex, and their stability is hard to determine.